A low-cost, wireless near-infrared spectroscopy device detects the presence of lower extremity atherosclerosis as measured by computed tomographic angiography and characterizes walking impairment in peripheral artery disease

Abstract

Background: Patients with peripheral artery disease (PAD) who experience intermittent claudication report a range of symptoms. Patients with symptoms other than classically described intermittent claudication may be at the highest risk for functional decline and mobility loss. Therefore, technologies allowing for characterization of PAD severity are desirable. Near-infrared spectroscopy (NIRS) allows for measurements of muscle heme oxygen saturation (StO₂) during exercise. We hypothesized lower extremities affected by PAD would exhibit distinct NIRS profiles as measured by a low-cost, wireless NIRS device and that NIRS during exercise predicts walking limitation.

Methods: We recruited 40 patients with PAD and 10 control participants. All patients with PAD completed a computed tomographic angiography, 6-minute walk test, and a standardized treadmill test. Controls completed a 540-second treadmill test for comparison. StO₂ measurements were continuously taken from the gastrocnemius during exercise. Variables were analyzed by Fischer's exact, χ², Wilcoxon rank-sum, and Kruskal-Wallis tests as appropriate. Correlations were assessed by partial Spearman correlation coefficients adjusted for occlusive disease pattern.

Results: Patients with PAD experienced claudication onset at a median of 108 seconds with a median peak walking time of 288 seconds. The baseline StO₂ was similar between PAD and control. The StO₂ of PAD and control participants dropped below baseline at a median of 1 and 104 seconds of exercise, respectively (P < .0001). Patients with PAD reached minimum StO₂ earlier than control participants (119 seconds vs 522 seconds, respectively; P < .001) and experienced a greater change in StO₂ at 1 minute of exercise (-73.2% vs 8.3%; P < .0001) and a greater decrease at minimum exercise StO₂ (-83.4% vs -16.1%; P < .0001). For patients with PAD, peak walking time, and 6-minute walking distance correlated with percent change in StO₂ at 1 minute of exercise (r = -0.76 and -0.67, respectively; P < .001) and time to minimum StO₂ (r = 0.79 and 0.70, respectively; P < .0001).

Conclusions: In this initial evaluation of a novel, low-cost NIRS device, lower extremities affected by PAD exhibited characteristic changes in calf muscle StO₂, which differentiated them from healthy controls and were strongly correlated with walking impairment. These findings confirm and expand on previous work demonstrating the potential clinical value of NIRS devices and the need for further research investigating the ability of low-cost NIRS technology to evaluate, diagnose, and monitor treatment response in PAD.

Keywords: Gardner-skinner protocol; Near-infrared spectroscopy; Peripheral artery disease; Six-minute walk.

Fig 1.

MOXY near-infrared spectrophotometer. The device measures 2.40 × 1.72 × 0.82 inches and houses four laser emitting diodes (wavelengths 680, 720, 760, and 800 nm), two incrementally spaced light detectors, and a lithium polymer batter with a 6-hour battery life. The device is attached to the lower extremity with a dual-sided adhesive tape before securing with Coban. A, Close up image of the MOXY NIRS probe. B, Application of MOXY to the lateral head of the gastrocnemius.

Fig 2.

Representative comparison of calf muscle oxygen saturation (StO₂) in control and peripheral artery disease (PAD) patients during exercise and recovery with designation of exercise variables. With standing from the seated baseline position both PAD and control participants experienced an initial increase in StO₂. With the initiation of exercise patients with PAD rapidly dropped below baseline to a minimum StO₂. Control participants experienced a gradual decrease in StO₂ throughout the duration of the exercise interval. All StO₂ are expressed as percent of baseline StO₂ for normalization. PAD is designated by the solid line and control by the dotted line. The green dotted line, Initiation of exercise; the red dotted line, exercise stop. 60s. StO₂ at 1 minute of exercise; BBT. time to first StO₂ below baseline; COT. claudication onset time; HT, time to maximum StO₂ during recovery interval; PWT, peak walking time; T₅₀, time to 50% recovery of baseline StO₂; T₁₀₀, time to 100% recovery of baseline StO₂; TTM, time to minimum StO₂.

Fig 3.

Correlation of resting ankle-brachial index (ABI) with peak walking time (PWT). Partial Spearman correlation coefficient adjusted for occlusive disease pattern.

Fig 4.

Correlation of percent decrease in oxygen saturation (StO₂) at 1 minute of exercise and peak walking time (PWT). Partial Spearman correlation coefficient adjusted for occlusive disease pattern.